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UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station. 

URBANA,  APRIL,  1902. 


BULLETIN  NO.  70. 


CANKER  OF  APPLE  TREES. 


BY  HEINRICH  HASSELBRING,  B.S.,  ASSISTANT  IN  VEGETABLE  PATHOLOGY. 


CANKER  DISEASES. — Throughout  the  fruit  growing-  region  of 
Illinois  bark  diseases  of  fruit  trees  are  common.  These  diseases 
are  popularly  known  as  "cankers."  Their  injury  consists  in  de- 
stroying more  or  less  extended  portions  of  the  bark  of  living-  trees, 
thereby  causing-  serious  wounds  which  interfere  with  the  nutrition 
of  all  parts  of  the  affected  limbs  above  the  canker  spot,  finally  re- 
sulting- in  the  death  of  the  limb  unless  the  tree  is  able  to  heal 
over  the  wound.  These  diseases  are  especially  dang-erous,  inas- 
much as  they  do  not  restrict  their  injury  to  a  single  crop  or  to  one 
season,  but  threaten  the  life  of  the  trees  themselves.  The 
diseases  are  mostly  perennial,  and  having-  once  g-ained  a  foothold 
they  progress  steadily  until  they  destroy  all  or  j  a  part  of  the  af- 
fected trees. 

The  term  "canker"  has  long-  been  in  use  in  England  to  desig- 
nate the  irregular  and  knotty  excrescences  resulting-  from  the  con- 

225 


226  BULLETIN  NO.  70.  \_ApriI, 

tinued  struggle  of  an  injured  part  of  a  tree  to  heal  up  wounds 
caused  by  various  agencies  as  sun-scald,  frost,  or  parasites.  In 
the  last  case  the  struggle  between  the  tissues  and  the  parasite 
continues  for  many  years  and  large  knotty  growths  are  formed  at 
the  wounded  spot.  The  European  canker  caused  by  Nectria  di- 
tissima  is  an  example  of  this  kind.  In  America  the  term  '  "canker" 
has  come  to  be  a  general  name  applied  to  all  diseases  involving 
more  or  less  extended  areas  of  the  bark,  although  these  diseases 
differ  widely  in  cause  and  in  the  effects  produced  on  the  host.  It 
is,  therefore,  a  generic  term  covering  a  wide  range  of  injuries.  So 
long  as  this  is  borne  in  mind  there  can  be  no  objection  to  its  use. 
Moreover  it  is  difficult  to  replace  popular  names  by  others  arbi- 
trarily chosen.  When,  however,  it  becomes  necessary  to  dis- 
tinguish between  different  cankers,  more  precise  names,  as  "New 
York  apple  tree  canker"  may  be  used. 

Some  of  the  common  cankers  caused  by  parasites  are  described 
below  in  order  to  enable  fruit  growers  to  distinguish  them  and  ac- 
quire a  more  accurate  knowledge  of  these  diseases.  This  report  is 
chiefly  concerned,  however,  with  a  new  canker  disease  which  is 
doing  serious  damage  in  the  apple  orchards  of  Illinois.  This 
canker  is  caused  by  a  fungus,  Nummularia  discreta,  Tul.,  which 
has  not  been  reported  as  a  parasite.  In  order  to  distinguish  this 
canker  from  others  it  may  be  designated  as  the  "Illinois  apple 
tree  canker." 

The  best  known  of  the  canker  diseases  is  that  caused  by 
Nectria  ditissima.  This  parasite  is  common  in  Europe  on  beech, 
apple  and  other  trees.  The  mycelium  kills  a  part  of  the  bark 
forming  cracks  which  are  partially  grown  over  by  the  neighboring 
tissues.  The  rapid  development  of  the  mycelium  prevents  com- 
plete healing  and  as  the  process  is  continued  year  after  year  large 
knots  or  cankers  are  produced.  In  these  the  wood  is  laid  bare  and 
shows  the  concentric  thick  ridges  caused  by  the  healing  tissues. 
This  canker  is  not  common  in  the  United  States. 

In  New  York  Paddock  investigated  a  bark  disease  which  he 
designated  as  the  New  York  apple  tree  canker.*  By  a  long  series 
of  careful  investigations  and  cross  inoculations  he  showed  this 
disease  to  be  due  to  the  fungus,  Sphceropsis  Mahritni,  Peck, 
which  causes  the  Black  Rot  of  the  apple.  The  appearance  of  the 
limbs  injured  by  this  fungus  is  various.  Paddock  describes  the 
diseased  limbs  as  having  dark  enlarged  sections  with  roughened 

*Paddock,  W.  The  New  York  Apple  Tree  Canker.  N.  Y.  Agr.  Exp.  Sta. 
Bui.  163  (1899)  "The  New  York  Apple  Tree  Canker"  (second  rep.)  do.  Bui.  185 
(1900). 


IQ02.J  CANKER   OF   APPLE   TREES.  227 

bark  and  portions  of  the  wood  laid  bare.  The  dead  bark  clings 
tenaciously  to  the  decaying-  wood.  This  canker  is  very  common  in 
Illinois.  The  fungus  attacks  nearly  all  parts  of  the  tree  from  the 
trunk  to  the  youngest  twigs.  The  bark  on  the  diseased  limbs  at 
first  assumes  a  dingy  brown  color  and  is  closely  appressed  to  the 
wood.  It  may  remain  in  this  condition  for  a  long  time,  some- 
times until  the  canker  spot  is  nearly  a  foot  in  length.  Around  the 
margin  of  the  spot  the  diseased  bark  is  slightly  depressed  and  is 
separated  from  the  sound  bark  by  a  distinct  line  and  often  by  a 
narrow  crack.  Cracks  and  rifts  appear  later  over  the  diseased  sur- 
face and  the  bark  assumes  the  dark  charred  appearance  character- 
istic of  the  canker. 

Stewart,  Rolfs,  arid  Hall*  have  suggested  the  possible  connec- 
tion of  Macrophoma  Malorum,  Berl.  &  Vogl.  and  of  a  species  of 
Cytospora  with  canker  diseases  of  apple  trees  and  pear  trees. 

The  Pacific  coast  canker  investigated  by  Cordley1  and  found 
by  him  to  be  due  to  Glceosporium  malicorticis,  Cordley,  does  not 
occur  in  this  state. 

THE  ILLINOIS  APPLE  TREE  CANKER, 

The  most  serious  canker  disease  in  Illinois  at  present  is  that 
caused  by  Nummularia  discreta^  Tul.  This  fungus  has  been  known 
for  a  long  time,  but  it  has  never  been  reported  as  being  the  cause 
of  a  disease.2  Its  usual  habitat,  like  that  of  a  species  closely  related 
to  it,  is  on  dead  wood.  As  a  saprophyte  the  fungus  has  been  fre- 
quently reported  both  from  Europe  and  America  on  dead  parts  of 
various  hosts.  It  occurs  on  the  wood  of  apple,  sorbus,  cercis, 
magnolia,  and  elm.  The  fungus  was  first  collected  in  America  by 
Schweinitz,  and  described  by  him  as  Sphczria  discreta.  He  fre- 
quently found  it  on  the  large  branches  of  apple  trees.  This 
disease  was  first  observed  in  this  state  during  the  past  summer, 
but  it  has  evidently  existed  here  for  some  time.  It  was  found  to 
be  widespread  through  the  southern  part  of  the  state  and  many 
individual  cankers  were  found  that  were  at  least  five  or  six  years 
old. 

*F.  C.  Stewart,  F.  M.  Rolfs  and  F.  H.  Hall.  AFruit  Disease  Su  rvey  of 
Western  New  York;  N.  Y.  Agr.  Exp.  Sta.  Bui.  191  (1900). 

'A.  B.  Cordley.  Some  Observations  on  Apple-tree  Anthracnose;  Bot.  Gaz. 
30:48.  Some  Preliminary  Notes  on  Apple-tree  Anthracnose.  Oreg.  Exp.  Sta. 
Bui.  60. 

'-It  would  seem  that  this  Nummularia  must  have  been  observed  as  a  parasite 
by  Tulasne,  who  states  (Sel.  Fung.  Carp.  II,  p.  46).  "It  grows  with  us  during  the 
autumn  and  winter  on  Sorbus  hybrida  L.  on  the  thick  bark  which  has  recently 
died." 


228  BULLETIN  NO.  70.  [April, 

The  following"  report  contains  the  results  of  some  observations 
and  investigations  made  on  this  disease  during-  the  past  summer. 
These  are  as  yet  in  no  way  complete,  but  some  suggestions  as  to 
remedial  measures  can  be  given  as  a  result  of  the  work.  As  the 
disease  is  a  more  serious  trouble  than  is  generally  supposed,  and 
also  to  furnish  an  answer  to  the  inquiries  of  some  fruit  growers 
who  have  become  alarmed  about  the  disease,  it  was  thought  best 
to  publish  the  results  thus  far  obtained. 

Inoculation  experiments  have  been  begun  but  it  is  yet  too 
early  to  form  any  conclusions  in  regard  to  them. 

DISTRIBUTION  OF  THE  DISEASE. 

The  disease  under  consideration  was  found  during  the  past  sum- 
mer in  all  the  apple  growing  regions  of  southern  Illinois. 
It  appears  to  be  most  severe  in  the  orchard  regions  near 
Neoga,  Salem  and  Centralia.  It  was,  however,  observed  also  in 
other  parts  of  the  state  and  it  is  probable  that  a  closer  examina- 
tion will  reveal  it  in  many  localities  which  it  has  been  impossible 
to  examine.  VVhen  fruit  growers  begin  to  recognize  the  canker 
better  a  more  accurate  knowledge  of  its  distribution  will  be  gained. 

Within  an  infected  orchard  its  distribution  is  usually  scattered, 
the  infected  trees  occurring  here  and  there  throughout  the 
orchard.  The  fact  that  it  does  not  seem  to  spread  evenly  from  a 
diseased  tree  to  all  the  neighboring  trees  would  lead  to  the  con- 
clusion that  its  spread  depends  upon  occasional  infections  under 
certain  favorable  conditions.  This  will  be  more  fully  considered 
in  the  following  pages: 

The  canker-wounds  are  usually  formed  on  the  large  limbs 
near  the  trunk  of  the  tree.  From  here  they  extend  upwards  on 
the  limb  and  frequently  down  into  the  trunk.  The  larger  limbs 
may  be  attacked  higher  up  in  the  tree,  however.  The  exact  position 
of  the  wound  depends  on  the  source  and  manner  of  infection. 

APPEARANCE  OF  THE  DISEASED  BRANCHES. 

The  appearance  of  the  canker  varies  greatly  with  age.  At  first 
the  canker  spots  are  inconspicuous  so  that  they  would  easily  be 
overlooked  by  the  casual  observer.  In  the  earliest  stages  observed 
the  diseased  bark  has  an  unhealthy,  dirty  brown  appearance.  It  is 
usually  depressed  a  little  below  the  living  bark.  The  spots  vary 
in  size,  being  sometimes  six  inches  in  diameter  at  this  stage. 
They  grow  most  rapidly  in  the  direction  of  the  long  axis  of  the 
limb.  If  the  interior  of  the  bark  be  examined  it  will  be  found  to 
have  a  mottled  appearance,  due  to  the  interspersion  of  sound  areas 


1902.] 


CANKER   OF   APPLE   TREES. 
PLATE    I. 


229 


230 


BULLETIN  NO.  JO. 

PLATE  II. 


{April, 


I0g2.j  CANKER   OF   APPLE   TREES.  23! 

within  the  dead  tissues.  The  boundary  between  the  dead  and  sound 
bark -is  sharply  marked.  Sometimes  cracks  appear  along-  this 
boundary.  The  dead  area  is  usually  a  little  depressed,  due  par- 
tially to  drying-  and  partially  to  the  increase  in  the  thickness  of 
the  sound  bark.  Often  the  wound  is  accompanied  by  a  flow  of  sap 
or  by  slime  flux.  This,  however,  is  probably  a  secondary  phe- 
nomenon not  due  directly  to  the  parasite. 

In  the  late  summer  or  autumn  the  fruiting-  stromata,  to  be 
more  fully  described  later,  appear  near  the  margin  of  the  diseased 
spot.  These  are  produced  under  the  bark,  which  soon  splits, 
forming-  star-shaped  ruptures  and  exposing-  the  pale  grayish  ochre, 
spore  cushions.  These  vary  from  one-eig-hth  to  one-fourth  of  an 
inch  in  diameter,  rarely  exceeding-  the  latter  limit.  When  the 
stromata  are  mature  they  have  the  form  of  a  more  or  less  irregu- 
larly circular  disc  somewhat  depressed  in  the  center,  or  even  cup- 
shaped.  As  they  are  formed  in  a  line  near  the  advancing-  margin 
of  the  canker  spot  those  of  the  different  years  come  to  lie  in  con- 
centric rows.  This  appearance  is,  however,  obliterated  in  the 
older  parts  of  the  canker. 

As  the  canker  spot  increases  in  size  it  chang-es  its  appearance. 
The  bark  of  the  older  parts  becomes  much  roug-hened  and  blackened 
as  if  it  had  been  charred.  Numerous  rifts  and  cracks  appear  over 
the  surface  of  the  dead  bark,  which  is  very  dry  and  brittle,  and 
falls  off  in  irreg-ular  patches  exposing-  the  dead  wood.  The  circu- 
lar stromata  are  firmly  attached  to  the  wood  by  means  of  a  ring-  of 
hard  fungous  tissue,  so  that  they  remain  seated  on  the  wood 
even  after  the  bark  has  fallen  away.  The  entire  blackened 
area  is  dotted  over  with  the  circular  stromata  which  form  the 
most  pronounced  distinguishing-  feature  of  this  canker.  The 
disease  is  always  easily  recog-nized  by  these  stromata,  which  dis- 
tinguish it  clearly  from  the  New  York  apple  tree  canker.  Plate 
I  shows  two  large  limbs  with  old  canker  spots. 

GROWTH  OP  THE  MYCELIUM. 

The  mycelium  evidently  extends  into  the  wood  of  the  tree 
where  it  grows  more  rapidly  than  in  the  bark.  Some  limbs  were 
examined  at  points  several  feet  away  from  the  canker  spot  and  the 
heart-wood  was  found  to  be  brown  and  discolored.  It  is  probable 
that  the  fungus  can  gain  an  exit  where  the  bark  has  been  broken 
and  form  new  canker  spots  at  such  points. 

The  effect  of  the  mycelium  on  the  wood  and  bark  has  not  yet 
been  fully  worked  out,  but  some  facts  may  be  noted  here.  The 
mycelium  kills  the  bark  wherever  it  penetrates,  and  finally  disinte- 


232  BULLETIN  NO.  JO.  \_April, 

grates  the  cells.  It  does  not  advance  evenly  through  the  tissues, 
but  leaves  islands  of  sound  tissue  in  the  midst  of  the  dead  bark. 
This,  of  course,  is  observable  only  in  the  young-  advancing-  part  of 
the  diseased  spot,  and  gives  this  area  the  mottled  appearance  al- 
ready mentioned.  Finally  these  spots  are  also  killed.  The  cells  of 
the  parenchtnya  tissue  and  medulary  rays  are  first  attacked.  The 
hard  bast  fibers  resist  the  action  of  the  fungus  and  can  be  seen  as 
glistening  groups 'of  cells  arranged  in  concentric  rings  in  the  midst 
of  the  dead  tissue.  In  the  recently  killed  tissue  numerous,  very 
delicate  hyaline  threads  of  the  fungus  can  be  seen  traversing  the 
cells  in  all  directions  and  sending  numerous  branches  into  the 
neighboring  cells.  The  boundary  between  the  sound  and  the  dead 
tissues  is  marked  by  a  very  delicate  layer  of  cork  cells  produced 
from  the  parenchmya  cells. 

INJURY  CAUSED  BY  THE  FUNGUS. 

The  injury  which  the  fungus  does  to  the  tree  is  at  first  only  local, 
being  restricted  to  the  area  of  the  canker  spot.  The  death  of  the 
tissues  here  is  due  to  the  direct  injurious  action  of  the  mycelium. 
The  rapid  advance  of  the  mycelium  prevents  the  formation  of  any 
thick  callous  or  wound-cork  on  the  part  of  the  tree.  As  a  result 
the  cankered  limbs  remain  comparatively  smooth  and  do  not  form 
the  knotty  excrescences  observed  in  the  case  of  nectria  canker.  In 
summer  a  very  narrow  layer  of  cork  cells,  not  visible  to  the  naked 
eye,  is  formed  on  the  boundary  line  between  the  dead  wood  and 
the  sound  tissues.  This  seems  to  be  the  only  effort  on  the  part  of 
the  tree  to  stop  the  advance  of  the  parasite.  As  the  canker  spot 
increases  in  size  the  limb  shows  the  indirect  effect  of  the  injury. 
The  parts  above  the  injury  begin  to  show  want  of  water  and  nour- 
ishment. The  leaves  assume  a  sickly  appearance  and  the  fruit  re- 
mains small.  The  growth  of  the  fungus  is  most  rapid  along  the 
direction  of  the  limb,  so  that  living  branches  may  sometimes  be 
diseased  for  a  distance  of  two  or  three  feet  before  they  are  finally 
and  completely  girdled.  Such  limbs  show  great  want  of  water 
and  the  fruit  produced  on  them  is  of  little  value.  When  a  limb  is 
completely  girdled  all  the  parts  above  the  canker-spot  die.  The 
death  of  limbs  usually  takes  place  in  late  summer  and  is  to  a  cer- 
tain extent  dependent  upon  other  conditions.  During  a  dry  season 
limbs  begin  to  suffer  and  die  even  before  they  have  been 
completely  girdled  ;  and  earlier  than  during  a  more  favorable 
season.  The  growth  of  the  fungus  may  continue  after  the 
death  of  the  infected  limb,  as  the  lower  part  is  still  in  un- 
interrupted connection  with  the  trunk.  Where  the  canker 


1902.]  CANKER   OF   APPLE   TREES.  233 

spot  is  near  the  trunk  it  rapidly  extends  down  the  stem  and 
threatens  the  life  of  the  tree  itself.  Plate  II  shows  a  condition 
common  in  Illinois  orchards.  One  of  the  largest  limbs  of  the  tree 
has  been  killed  by  the  canker-fungus. 

How  THE  FUNGUS  ENTERS  THE  TREE. 

So  far  as  observations  go  at  present  they  seem  to  show  that 
Nummularia  discreta,  like  Nectria  and  other  fungi  of  similar 
habits,  is  a  wound-parasite.  Apparently  it  is  unable  to  gain  an 
entrance  into  the  tree  except  through  a  previously  existing  wound 
in  the  bark.  The  spores  which  originate  in  the  stromata  germ- 
inate readily  in  water,  hence  when  they  lodge  on  the  moist  sur- 
face exposed  by  a  wound  their  germ-tubes  easily  effect  an  entrance 
into  the  tissues  of  the  host.  Within  the  tissues  the  mycelium  ad- 
vances more  rapidly  in  the  wood  than  it  does  in  the  bark. 

A  knowledge  of  the  manner  in  which  the  parasite  enters  its 
host  forms  the  best  foundation  for  combating  diseases  of  this 
nature.  Too  much  attention  cannot  be  given  to  the  various  causes 
by  which  wounds  are  produced  in  trees.  One  of  the  most  promi- 
nent sources  of  infection  was  found  to  be  through  wounds  neces- 
sarily caused  by  pruning.  Too  much  stress  cannot  be  laid  on  the 
proper  pruning  of  trees;  but  even  if  a  branch  is  removed  without 
leaving  a  stub,  the  wound  still  offers  a  good  point  of  infection  for 
the  fungus.  Yet  such  wounds  heal  rapidly  and  are  less  dangerous 
than  the  long  stubs  too  frequently  observed  in  the  orchards  of 
Illinois. 

In  one  well  cultivated  orchard  it  was  a  noticeable  fact  that 
most  of  the  canker  spots  were  situated  on  the  under  side  of  the 
large  limbs  near  the  trunk.  In  this  case  it  is  probable  that  the  in- 
juries were  caused  by  some  part  of  the  harness  in  cultivating  the 
orchard.  Often  the  original  wound  was  entirely  obscured  by  the 
subsequent  action  of  the  canker.  It  must,  however,  be  borne  in 
mind  that  the  first  injury  which  affords  an  infection-court  may  be 
very  slight  and  easily  obscured.  Wounds  caused  by  pickers  who 
injure  the  bark  of  the  trees  by  climbing  about  in  them  also  offer 
favorable  points  for  infection.  A  case  of  this  kind  occurred  in  an 
orchard  in  Neoga,  where  many  of  the  cankers  could  be  traced 
to  such  injuries. 

The  foregoing  cases  represent  several  classes  of  injuries  which 
are  likely  to  occur  in  orchards.  Of  course  any  kind  of  injury  such 
as  breaking  of  limbs,  twisting  and  splitting  of  branches,  and  per- 
haps insect  injuries  will  furnish  an  infection-court  for  fungi. 


234  BULLETIN  NO.  jo.  [April, 

PREVENTION  AND  TREATMENT. 

From  the  nature  of  the  attacks  of  Nnmmularia  discreta  there 
seems  to  be  no  method  of  curing-  the  injury  after  the  parasite  has 
once  gained  entrance  into  the  limb.  The  mycelium  extends  through 
the  wood  some  distance  beyond  the  injured  spot.  It  is  thus  well 
protected  within  the  heart-wood  of  the  tree.  If  the  canker 
is  found  in  its  first  stages,  however,  it  may  be  useful  to  cut 
away  the  injured  bark  and  a  portion  of  the  wood  and  keep  the 
wound  covered  with  Bordeaux  mixture  or  paint.  The  bark  at  the 
edges  of  the  wound  will  grow  out  and  heal  over  the  injury. 
Limbs  which  have  extended  diseased  areas  should  be  removed  and 
burned.  When  a  limb  is  badly  injured  it  is  so  weakened  that  the 
fruit  borne  on  it  is  of  little  value  and  the  death  of  the  limb  is  only 
a  question  of  time.  Meanwhile  every  diseased  spot  is  a  source  of 
danger  to  the  orchard  and  cannot  be  too  quickly  removed.  The 
canker  spots  frequently  occur  near  the  trunk  so  that  the  life  of  the 
tree  is  endangered  by  them.  Infections  through  wounds  caused 
by  pruning  can  be  to  a  great  extent  prevented  by  careful  attention 
to  the  details  of  the  operation.  The  indiscriminate  and  careless 
cutting  of  branches  frequently  practiced  in  this  state  is  detrimental 
to  the  health  of  the  tree  aside  from  the  opportunities  it  offers  for 
parasites  of  all  kinds  to  enter.  Long  stubs  should  never  be  left  on 
the  tree.  Limbs  should  be  cut  close  to  the  parent  branch  without 
making  the  wound  unnecessarily  large.  Wounds  caused  by  proper 
pruning  heal  without  difficulty.  They  should  be  kept  painted  or 
covered  with  Bordeaux  mixture.  Injuries  caused  by  climbing 
about  in  the  trees  can  be  prevented  entirely  by  picking  from  lad- 
ders. There  is  rarely  any  necessity  for  climbing  the  tree  to  pick 
the  fruit. 

MINUTE  CHARACTERS  OF  THE  FUNGUS. 

The  young  stromata,  as  has  been  stated,  appear  beneath  the 
surface  of  the  bark.  They  originate  as  circular  patches  of  dark 
colored  tissue  formed  in  the  bark  from  numerous  closely  inter- 
woven fungous  threads,  Plate  III,  Fig.  A.  One  of  these  is  called  a 
stroma.  On  the  upper  surface  of  the  stroma  are  numerous  erect 
threads  which  have  very  small  spore-like  bodies  or  conidia.  The 
conidial  layer  is  at  first  covered  by  the  epidermis  intergrown  with 
fungous  tissue,  and  is  not  exposed  until  this  fleshy  epidermal 
layer  ruptures.  The  conidia  are  small,  one-celled.  Attempts  to 
germinate  them  have  been  unsuccessful.  Tulasne*  figures  germi- 

*Plate  V,  1.  c. 


1902.] 


CANKER   OF   APPLE   TREES. 
PLATE    III. 


235 


c 


236 


BULLETIN  NO.  JO. 


[April, 


PLATE  IV. 


1902.]  CANKER   OF   APPLE   TREES.  237 

nating  conidia  of  this  species,  and  Bref  eld*  describes  the  germina- 
tion of  the  conidia  of  N.  lataniaecola. 

As  the  stroma  grows  older  a  ring  of  black  stromatic  tissue  is 
formed  beneath  the  disc.  This  ring  extends  through  the  bark  into 
the  wood  of  the  limb.  The  stromatic  tissue  is  hard  and  resistant, 
and  remains  in  place  after  the  bark  around  it  has  broken  loose. 
In  sectional  outline  the  whole  stroma  is  somewhat  cup-shaped. 
(Fig.  B).  The  ring  of  tissue  can  be  seen  as  a  black  circular  line 
in  the  wood  when  the  stromata  are  cut  off.  The  upper  layer  of 
the  stroma  at  this  time  contains  numerous  flask-shaped  cavities 
called  perithecia,  with  long  necks  opening  at  the  surface.  (Fig.  C). 
The  spores  of  the  fungus  originate  within  long  sacks  or  asci  in 
these  cavities.  (Fig.  D). 

The  spores  are  nearly  round  or  slightly  oblong.  They  are 
rather  large  and  have  a  thick  brown  membrane.  Along  one  side 
a  lighter  line  can  be  seen  indicating  the  place  where  the  spore 
membrane  will  rupture  at  germination.  The  spores  are  expelled 
from  the  sacks  and  come  to  lie  in  little  black  heaps  on  the  surface 
of  the  stromata.  It  is  stated  by  Tulasne  that  the  spores  are  ex- 
pelled in  March  or  April.  They  may  be  found  on  the  discs  at 
almost  any  time.  It  is  probable  that  many  cling  there  for  a  year 
or  more. 

Spores  taken  from  specimens  collected  in  September  germ- 
inated readily  when  sown  in  water  or  in  beet  infusion.  If  these 
had  been  expelled  from  the  perithecia  in  spring  they  evidently  re- 
tained their  vitality  all  summer  and  for  several  months  more  in 
the  laboratory.  In  germination  the  exospore  cracks  along  the 
lines  previously  described.  Two  germ  tubes  originate  from  the 
endospore.  These  turn  away  from  each  other  and  remain  at  first 
closely  appressed  to  the  spore.  Then  they  grow  out  in  opposite 
directions.  Germinating  spores  are  shown  on  Plate  IV. 

The  germination  of  the  spores  is  greatly  influenced  by  their 
supply  of  air  or  oxygen.  When  many  spores  were  sown  in  a  drop 
of  liquid  those  which  were  near  the  margin  germinated  readily  in 
18  to  20  hours,  but  those  which  were  submerged  grew  tardily  or 
not  at  all.  In  the  same  manner  when  the  drop  was  spread  out  over 
the  slide  the  spores  germinated  freely.  This  may  be  of  significance 
in  connection  with  the  natural  coditions  under  which  the  spores 
must  germinate  and  grow. 

In  some  instances  the  young  mycelium  before  it  had  grown  to 
any  considerable  size  formed  a  cluster  of  branches  near  the  apex, 


*Brefeld.     Untersuchungen  X.-256. 


238  BULLETIN    NO.   70.  [April, 

and  these  produced  conidia  resembling-  those  originating-  on  the 
young-  stroma.  (Fig-.  C).  More  frequently  the  threads  continue 
to  grow  and  branch  freely,  forming-  a  very  compact  colony  of 
mycelium.  The  growth  of  the  colony  is  very  slow  on  the  media 
tried.  Often  long-  threads  grow  out  from  the  colony  and  at  their 
end  form  a  dense  brush  of  branches.  None  of  the  colonies 
fruited  in  artificial  cultures. 

SUMMARY. 

The  common  term,  "canker,"  includes  all  diseases  involving- 
portions  of  the  living  bark  of  trees.     These  may  be  due  to  differ 
ent  causes.     Pp.  225-226 

Several  canker- diseases  caused  by  funguous  parasites  are 
known  in  the  United  States.  The  most  common  diseases  of  this 
kind  in  Illinois  are  the  New  York  apple  tree  canker  caused  by  the 
Black  Rot  fungus,  and  the  Illinois  apple  tree  canker  caused  by 
Nummularia  discrcta,  Tul.  Pp.226-227. 

The  Illinois  apple  tree  canker  occurs  on  the  larg-e  limbs  and 
trunks  of  apple  trees.  It  produces  extended  blackened  areas 
within  which  the  bark  cracks  and  finally  crumbles  away.  This 
always  results  in  the  death  of  the  affected  limb.  Pp.  227-233. 

The  fung-us  causing1  the  canker  is  a  wound  parasite  g-aining- 
entrance  into  the  tree  through  wounds  caused  by  pruning  or  by 
accidental  injuries.  P.  233. 

The  best  means  of  preventing-  the  disease  is  to  avoid  as  much 
as  possible  all  injury  to  the  bark,  to  prune  properly,  and  to  paint 
wounds  with  an  antisceptic  solution.  Badly  diseased  limbs  should 
be  cut  and  burned.  P.  234. 


CANKER  OF  APPLE  TREES.  239 

EXPLANATION  OF  PLATES. 

Plate  I.,  page  229.  (A)  An  old  diseased  limb,  4%  inches  in  diameter,  showing  the 
stromata  of  the  canker-fungus.  (B)  Another  somewhat  larger  limb 
showing  earlier  stages  of  the  disease.  The  limb  was  infected  through 
the  wound  caused  by  pruning  away  a  branch.  The  progress  of  the 
disease  is  marked  by  the  concentric  lines  of  young  stromata.  At 
the  base  of  the  limb  the  boundary  between  the  dead  and  the  living 
bark  can  be  distinguished. 

Plate  II.,  page  230.  Shows  a  tree  having  one  of  the  main  branches  killed  by  the 
canker. 

Plate  III.,  page  235.  (A)  A  young  stroma  of  the  canker-fungus.  (B)  A  fully 
developed  stroma,  showing  the  perithecia  sunk  in  the  upper  layer. 
(C)  One  of  the  perithecia  more  enlarged  to  show  the  way  in  which 
the  spores  are  borne  and  expelled.  (D)  Two  of  the  sacs  or  asci 
containing  spores.  The  spores  will  escape  through  the  pore  at  the 
upper  end  of  the  sac. 

Plate  IV  ,  page  236.  (A)  Germinating  spores  (X 600).  (B)  Spores  more  advanced 
in  germination  (X8oo).  (C)  Condia  borne  on  the  young  mycelium 
(X6oo).  (D)  Mycelium  from  a  culture  36  hours  old,  1X385).  These 
threads  grow  through  the  wood  and  bark,  causing  the  death  of  the 
tissue. 


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